Many different types of heating units are used in residential and commercial buildings to heat the interior of those buildings. One of these different types of heating units is a forced air gas-fueled unit. Frequently, these units are located centrally within the building and duct work extends to registers positioned throughout the building. These units include a burner for heating air drawn into the unit and a fan or blower for forcing the heated air through the duct work to deliver the air to the registers. Usually, some type of heat exchanger is used to heat the air so that the heated air and combusted gases do not mix. Because the combusted gases from the burner include high concentrations of carbon monoxide which are hazardous to humans, circulating the combusted gases throughout the building is not desirable.
These centrally-located, forced-air, gas-fueled heating units are highly efficient and work well for many applications. However, in some applications the heaters are not desirable. For example, in hotels and motels it is desirable to permit the temperature in each room to be individually controlled as each guest may be comfortable when the air is within a different temperature range. In order to achieve widely varying temperatures from room to room, separate heater units are frequently employed. Further, because the size of a hotel room or suite is typically not as large as an entire house, the relatively large centrally located furnaces used in houses are too large for use in individual hotel rooms. Thus, smaller heaters are desirable in hotel rooms. These smaller heaters are compact, and are generally designed to be positioned against an exterior wall of the room to maximize the useable floor space in the room. As a result, these smaller heaters are commonly referred to as "wall heaters".
Another example where smaller heaters are desirable is in additions to existing buildings. For small additions, it is frequently uneconomical to re-route and/or add onto the existing duct work. Further, sometimes even when the duct work could be re-routed economically, the added load on the existing furnace would be so great as to prevent it from effectively heating the building. Thus, rather than re-route the existing duct work or replace the existing furnace, it is sometimes desirable to use a smaller second furnace in additions to existing buildings.
Typically forced-air, gas-fueled wall heaters are comprised of a cross-flow heat exchanger, a blower positioned to force air from the room past pipes in the heat exchanger, and a burner for heating air flowing through the pipes. In addition, most wall heaters include various control systems and sensors which regulate the heater and shut down operation when the sensors measure certain undesirable conditions. Prior art heater units usually include only one blower which is generally directed to force air over the central portion of the heat exchanger. The heat exchangers in these units may take one of several different configurations. Typically, however, the exchangers include a mixed stream flowpath and an unmixed stream flowpath. As the name suggests, the mixed stream flowpath is configured to permit the air to circulate as it travels through the exchanger so that the air emerges from the exchanger at a uniform temperature. In contrast, the unmixed stream flowpath is configured to inhibit the air from mixing. The burner is usually placed in series with the unmixed stream flowpath and the air from the room is usually forced along the mixed stream flowpath. Thus, the combusted gases travel through the unmixed stream flowpath and the heated air travels through the mixed stream flowpath and emerges at a uniform temperature.
Regardless of the actual configuration used, wall furnaces are more desirable when they are more efficient, less expensive and smaller. The ever increasing cost of energy and the highly competitive nature of the HVAC industry drive heater manufacturers to constantly seek to improve the efficiencies of their heaters. Higher heater efficiencies reduce fuel consumption thereby reducing the consumer's heating costs and improving their salability. As with most consumer goods, the less expensive they can be manufactured without compromising effectiveness, durability, and quality, the more desirable the product is to the purchasing public. Therefore, the less expensive a manufacturer can make a heater without sacrificing quality and efficiency, the better. Finally, because the space in hotel rooms and new construction is at a premium, the smaller a heater unit can be made, the more desirable it is.